(a) Field of the Invention
The present invention relates to a method for tracking carrier frequency offsets and sampling frequency offsets. More specifically, the present invention relates to a method and device for tracking carrier frequency offsets and sampling frequency offsets in an orthogonal frequency division multiplexing (OFDM) wireless communication system.
(b) Description of the Related Art
In general, subcarriers that have orthogonality are modulated by digital data and concurrently transmitted in the OFDM system. Since the summation of the modulated subcarriers in a narrowband forms a total transmission band, symbol intervals on the time axis increase, which reduces inter-symbol interferences caused by multipath fading.
However, in the like manner of other wireless communication systems, the OFDM system causes lowering of performance because of differences of carrier frequencies at a transmitter and a receiver. That is, offsets of the carrier frequencies cause a reduction in the SNR (signal to noise ratio) and generate intercarrier interference, thereby deteriorating system performance.
Also, the difference between the sampling frequencies at the transmitter and the receiver generates offsets at the sampling time, which causes lowering of performance of the OFDM system.
As described above, the carrier frequency offsets and the sampling frequency offsets not only lose the orthogonality of the OFDM system, but also result in deterioration of performance of the system. Therefore, it is required for the receiver to continually track the offsets of the carrier frequencies and the sampling frequencies and compensate for them.
In the compensation process, phase offsets caused by the offsets of the carrier frequencies and the sampling frequencies need to be tracked, and methods for tracking them include a method for using information of previously known data such as a pilot signal, and a method for restoring payload data and using the restored data.
Further, a related paper entitled “Carrier frequency acquisition and tracking for OFDM systems,” by M. Luise and R. Reggiannini in the IEEE Trans. Communications, October 1996, pp, 1590-1598 proposed a method for tracking carrier frequency offsets and compensating for them in the time domain on the basis of demodulated data and signals received from a transmitter.
However, the method for using the received signals (e.g., pilot signals) allows more accurate tracking by using information previously known by the receiver, but if the pilots are fixedly located on a predetermined restricted number of subchannels, and the channel gain of the subchannels is low, reliability of an estimate also becomes low.
Another related paper, entitled “Timing recovery for OFDM transmission,” by B. Yang in the IEEE Journal on Selected Area in Communications, November 2000 proposed a method for estimating sampling frequency offsets and compensating for them using delay characteristics of channel power.